"Everybody wins" would involve everyone walking away with equal or more than they had at the time of parting, which could be done by opening the source of the product instead of letting it die of bitrot.
They owed them the honesty of saying at the start:
"We never intend to make this an actual product. This is only here to get us attention and hired. If you use this, be aware we'll kill it as soon as anyone hires us."
But who's to say that's what they intended at the start? They may well have entirely expected to make it a successful startup, only to find themselves unable to make the profit required to do so, so they got acquihired.
If you wanted to use an app with open source and a permanent future you should have installed an open source app. Installing a closed source app created by a business and depending on them open sourcing it when the business closes is folly.